Metallic neodymium and neodymium-iron alloys, which currently find increasing use as materials for the manufacture of permanent magnets, e.g. the neodymium-iron-boron alloys discussed in German Patent Disclosure DE-OS 37 29 361, DRAXLER et al., can be made by electrolytic reduction of salt melts containing neodymium compounds. If it is desired to recover the neodymium-iron alloys, the electrolysis cell uses iron cathodes which are consumed to provide the iron.
In their 1968 report cross-referenced above, E. Morrice and co-authors suggest obtaining neodymium and neodymium-iron alloys under an inert atmosphere from a melt of 50 Mol-% lithium fluoride, 50 Mol-% neodymium fluoride, and neodymium oxide dissolved therein, using graphite anodes, and insoluble tungsten or molybdenum cathodes, or consumable iron cathodes.
Japanese Patent Application 02-004,994 (abstracted in Chemical Abstracts, Vol. 112, Abstract No. 225 539p, 1990) describes the electrolysis of melts of 65.9 weight-% (20 Mol-%) neodymium fluoride and 34.1 weight-% (80 Mol-%) lithium fluoride and of 2 weight-% neodymium oxide, 64.6 weight-% (20 Mol-%) neodymium fluoride and 33.4 weight-% (80 Mol-%) lithium fluoride with carbon anodes and carbon or iron cathodes. For consumption of the carbon powder generated during the electrolysis and floating to the melt surface, the melt is electrolyzed in an oxygen-containing atmosphere.
European Patent Application No. 0 177 233 B1 relates to the manufacture of neodymium-iron alloys by electrolysis of melts. Under a protective gas atmosphere, a melt or bath containing 35-76 weight-% neodymium fluoride, 20-60 weight-% lithium fluoride, 0-40 weight-% barium fluoride, and 0-20 weight-% calcium fluoride is electrolyzed with at least one carbon anode and at least one iron cathode. The neodymium separating out at the iron cathode reacts with the iron to form an alloy, and the resulting neodymium-iron alloy (which is fluid at the temperature of the bath) drops from the cathode into a container below. The electrolysis occurs at 770.degree.-950.degree. C. on application of direct current to the carbon anode with a current density of 0.05-0.60 A/cm.sup.2 and to the iron cathode with a current density of 0.50-55 A/cm.sup.2.
With progressively increasing duration of electrolysis, the carbon anodes, used in this conventional method, are eroded by oxidation, so that they must be continually adjusted, and replaced at regular intervals. The destruction of the anodes also contaminates both the molten bath, and the neodymium-iron alloys generated, with carbon and with any other impurities present in the anode material. Oxides and fluorides of the carbon escape into the atmosphere surrounding the electrolysis cell, and can raise environmental concerns.
Accordingly, it is an object of the present invention to provide anodes, for such an electrolysis method, which, compared to carbon anodes, are consumed less quickly and possess an improved chemical resistance to the molten baths. This will have the advantage of producing neodymium and neodymium-iron alloys of increased purity, which are needed in the manufacture of permanent magnet materials.